Recent image signal sources range over a variety of sorts. Especially in computer displays, many kinds of scanning frequencies are used, and there are cases using a plurality of scanning frequencies in a single computer. Therefore, display apparatuses are required to have a multi-scanning deflection system. On the other hand, display screens have come to diversify in size. Especially, the demand for large-scale displays is increasing progressively, and display of computer signals by a large-scale projector has become active.
However, if the multi-scanning system is employed in a large-scale display apparatus, a lot of problems must be overcome, because of the deflection circuit being complex and the cost of component parts being high, and from the viewpoint of ensuring the reliability against an increase in temperature inside the device, among others. An existing scheme is to fix the deflection frequency to a single frequency, such as 60 Hz/31.5 kHz, and to process and convert various sorts of image signals into image signals of the single scanning frequency.
An example of such systems is schematically shown in FIG. 1. Introduced into a frequency converter 40 are three kinds of image signals different in scanning frequency, for example, ordinary NTSC signal (field frequency 59.9 Hz and horizontal scanning frequency 15.734), SVGA computer signal whose field frequency ranges from 50 Hz to 70 Hz and whose horizontal scanning frequency ranges from 35 kHz to 50 kHz, and HDTV signal having the field frequency of 60 Hz and the horizontal scanning frequency of 33.75 kHz. The frequency converter 40 converts the input image signals into image signals of a single scanning frequency 60 Hz/35.5 kHz, and supplies these image signals to a display apparatus.
Nowadays, display apparatuses using pixel-fixed devices are prevailing widely. Since the number of pixels is fixed, they need scanning conversion when displaying image signals of various modes. FIG. 2 shows a system in which an NTSC signal with 240 scanning lines, SVGA signal with 600 scanning lines and HDTV with 518 scanning lines are scan-converted by a frequency converter 40. Scan-converted image signals are displayed on a display apparatus, such as plasma display apparatus, having fixed pixels of 640 dots by 480 lines.
Next explained is a process of scan conversion by the frequency converter 40. FIG. 3 is a block diagram showing a main construction for scan conversion which has been used to date. The "scan conversion" pertains to converting, for example, a SVGA signal (800 pixels by 600 lines) into a VGA signal (640 pixels by 480 lines).
As shown in FIG. 3, an input image signal 50 is converted into a digital signal by an A/D converter 26, and provisionally stored in a field memory 20 which is configured to write data of the effective image period in one horizontal (1H) period. An independent sync signal asynchronous with the input image signal 50 is created from clock pulses 62 by a sync signal generator 38. An image signal is read out from the field memory 20 by clock pulses 62, and it is converted into an analog signal by a D/A converter 36. A primary processing circuit 27 is provided on the write side of the field memory 20, and a secondary processing circuit 37 is provided on the read side thereof.
The A/D converter 26, in which the sampling frequency is determined appropriately, executes A/D conversion so that one horizontal period be a predetermined number of sampling, for example, 800, and the effective pixels in one horizontal period be 640 pixels. The primary processing circuit 27 conducts vertical interpolation processing to convert, for example, 600 lines into 480 lines. The secondary processing circuit 37 adds a horizontal sync signal and a vertical sync signal.
A PLL circuit 21 is provided to generate a sampling clock for the A/D converter 26. The PLL circuit 21 includes a phase comparator 22, low pass filter (LPF) 23, voltage control oscillator (hereinafter abbreviated to VCO) 24, and frequency divider 25. The PLL circuit 21 generates a clock pulse 52 synchronous with the horizontal sync signal 51 of the input image signal 50. A/D conversion or other processing prior to clock re-riding is conducted by the clock pulse 52. The secondary processing circuit 37 uses a clock pulse 62 output from a free-run clock generator 39. The clock generator 39 is a quarts oscillator which stably generates clock pulses. The output side sync signal is made in the sync signal generator 38 by using the clock pulse 62, and does not synchronize with the input side sync signal.
The scan converting device explained above functions to adjust the horizontal size of the display screen. In case of display outputs of computers, not only the length of one horizontal period (horizontal frequency) but also the length of the effective image period in one horizontal period are inconstant. Therefore, in outputs from the A/D converter 26, the number of pixels corresponding to the effective image period is not always a predetermined number (for example, 640). As a result, when an output image signal is displayed on the display apparatus, the display apparatus often fails to display the full extent of the effective image period but often drops edge portions thereof. Since representation of panels, icons, and so fourth, typically lies along edges of computer displays, failure to represent these things degrades the manipulative performance. To overcome the problem, computer displays are configured to permit a user to appropriately adjust the horizontal width of the display screen in addition to the scan conversion. This adjustment is called horizontal size adjustment.
In beam deflection systems like a CRT display as shown in FIG. 1, the horizontal size is adjustable by adjusting the amplitude of horizontal deflection. However, pixel-fixed displays like that shown in FIG. 2 cannot use the same method for horizontal size adjustment, and relies on signal processing for horizontal size adjustment. CRT displays using beam deflection can employ horizontal size adjustment by signal processing as well.
The scan conversion device shown in FIG. 3 can adjust the horizontal size. More specifically, by introducing an external UP/DOWN signal increasing or decreasing the frequency division ratio into the frequency divider 25 and by changing the ratio of oscillation frequency of VCO 24 relative to the horizontal scanning frequency of the input image signal, the number of sampling in one horizontal period of the input image signal can be changed. On the other hand, the number of sampling in one horizontal period at the output side (read-out side) is held always constant. For example, if the frequency division ration of the frequency divider 25 is 1/800, then the number of sampling of one horizontal period of the input image signal is 800. If the frequency division ration is made larger than 1/800, the frequency of the sampling clock becomes high, the number of sampling of one horizontal period becomes larger than 800, and the number of pixels in the effective image period increases.
In this manner, in the construction shown in FIG. 3, the number of pixels in the effective screen relative to the sampling number of one horizontal period is changed by adjusting the frequency of the sampling clock of the A/D converter 26. As a result, the length of the effective image period relative to one horizontal period is changed, and the horizontal size can be adjusted correlatively.
This method for horizontal adjustment fixes the sampling number of one horizontal period on the output side always constant, and changes the number of pixels in the effective image period per one line of the input image signal. As a result, when the horizontal size is changed, the number of pixels relative to the effective screen of the input image signal also changes. That is, horizontal size adjustment invites a change in sampling point (sampling phase) for sampling by the A/D converter 26, and deteriorates the quality of images in case of signals with a high horizontal frequency (for example, signals of fine characters). Since typical computer signals are for representation of fine characters, this is a serious problem. Moreover, since the number of pixels in the effective screen changes, the field memory 20 for synchronous re-riding is required to have a much larger capacitance to allow for a margin beforehand.
It is therefore an object of the invention to provide a scan converting device and a scan converting method capable of horizontal size adjustment without deteriorating the quality of images on the display screen in a device configured to convert scanning frequencies for display of images.